The origin of cratonic diamonds — Constraints from mineral inclusions

Abstract

The origin of cratonic diamonds is reviewed on the basis of nearly 5000 analyses of silicate, oxide and sulphide inclusions in diamonds. Compositional fields are defined for common minerals of the peridotitic, eclogitic and websteritic inclusion suites and used to establish the characteristics of diamond source rocks in the subcratonic lithospheric mantle. Peridotitic inclusion compositions overlap with the record established from cratonic garnet peridotite xenoliths and xenocrysts but reflect overall higher levels of depletion in basaltic components. The interior of the Kaapvaal Block (Kalahari Craton) is by far the best studied diamond source region in the world but appears to be unique rather than representative because of extreme levels of chemical depletion preserved in the peridotitic inclusion suite. Major and trace element characteristics of peridotitic diamond sources indicate polybaric melt extraction proceeding from the garnet into the spinel stability fields, most readily explained by protolith depletion in Archean mid-oceanic ridge environments. Eclogitic mineral inclusions broadly reflect basaltic source compositions and show chemical trends that are indicative of igneous fractionation and cumulate enrichment in magmatic precursors. In agreement with mounting evidence from xenolith studies, eclogitic diamond sources are linked to subducted oceanic protoliths. A more mafic character relative to present-day MORB may relate to (i) higher degrees of partial melting in Archean or early Proterozoic spreading centres and (ii) secondary melt depletion during subduction, or after emplacement in the subcratonic lithosphere. In line with a subduction origin of eclogitic diamond source rocks, mines with predominantly eclogitic diamond populations are generally (but not invariably) associated with craton margin settings or lithosphere with a post-Archean tectonothermal history. The websteritic suite is poorly defined and reflects a range of broadly pyroxenitic source rocks intermediate between peridotite and eclogite. Geothermometry, based on inclusions and nitrogen aggregation in diamonds, indicates that crystallization and mantle storage of peridotitic, eclogitic and websteritic diamonds occurred under the same thermal conditions. Geobarometry for peridotitic inclusions shows that the majority of diamonds formed at depths of less than 200 km along model geotherms corresponding to 38 to 42 mW/m 2 surface heat flow. Lower geothermal gradients observed for diamonds from the Kalahari and Slave cratons likely represent re-equilibration of touching inclusion pairs to cooling ambient conditions, suggesting that diamond formation was accompanied by transient heating events. Diamond precipitation is interpreted to have occurred during metasomatic events under super-solidus (melt dominated) and sub-solidus (CHO-fluid dominated) conditions. Increasing evidence for a reduced character of the subcratonic lithospheric mantle implies that diamond precipitation through redox reactions requires upward migration of carbonate-bearing melts/fluids. In such a redox scenario high solubility of sulphate relative to sulphide in melts/fluids may provide an explanation for a high abundance of sulphide inclusions as a consequence of co-precipitation with diamond in response to decreasing oxygen fugacity. Such comparatively oxidized metasomatic agents cannot derive from the reduced deep upper mantle and, therefore, likely relate to recycling of oceanic lithosphere.